Propeller sound field modification systems and methods

ABSTRACT

A propeller system for an aircraft includes an assembly for modifying a sound field of the propeller system. The propeller system includes a rotor supported for rotation about a rotor axis. The rotor has a central hub and a plurality of blades each extending outwardly from the hub to a tip. The rotor and blades are operable to propel an aircraft to travel in a direction. The rotor blades define a rotor plane perpendicular to the rotor axis. The blade tips define a circumferential rotational path as the blades are rotated by the rotor. The propeller system includes an acoustic resonator or multiple resonators having openings disposed within a distance to the propeller blade tip that is small compared to the wavelength of the propeller&#39;s fundamental blade tone and proximate to the rotor plane. The resonators are excited by tip flow of the blade as it passes the opening. The acoustic resonators are configured and positioned so as to direct acoustic energy to modify the sound field of the propeller system at blade pass or higher harmonic frequency tones in a desired direction relative to the aircraft.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.13/919,595, filed on Jun. 17, 2013, pending, and claims the benefit ofU.S. Provisional Patent Application No. 61/802,205, filed on Mar. 15,2013, both of which are hereby incorporated by reference.

GOVERNMENT LICENSE RIGHTS

This invention was made with United States government support under U.S.Navy SBIR Award No. N68936-11-C-0017. The government has certain rightsin this invention.

TECHNICAL FIELD

The present disclosure relates generally to propeller sound fieldmodification and, more specifically, to systems and methods formodifying the sound field of an aircraft propeller.

BACKGROUND

It is common knowledge that propeller systems, for example, aircraftpropeller systems, pushers, tractors, and tail rotors, generate a noisesignature. It is generally desirable to reduce the noise signature ofpropellers. As noted in U.S. Pat. No. 7,992,674, the disclosure of whichis incorporated herein, noise generated by propeller systems has bothbroadband and tonal components. Tonal noise results from propeller bladeinteractions with time-invariant flow distortions. When spectrallydominant, blade tones are of primary concern in noise controlapplications due to their particular annoyance.

The noise problem is particularly acute for surveillance aircraft anddrones that must avoid detection or creating a nuisance near populatedareas. Several conventional techniques have been used to reducepropeller noise, but they all tend to reduce the efficiency of thepropeller. Some conventional noise control techniques include,increasing the number of blades, slowing the rotating speed of thepropeller, and shaping the blades for minimum noise creation.

Prior approaches used to reduce blade tone sound pressure levels (SPLs)have utilized both active and passive noise control methods. Passiveblade alterations, such as rotor/stator spacing in axial fans, leaning,sweeping or contouring, numbering, and irregular circumferential bladespacing, have been demonstrated effective for fan noise reduction. Fewpassive approaches have demonstrated the ability to reduce blade tonenoise locally in the blade region with minimal impact on fan efficiency.

The concept of noise cancellation by introducing secondary sources orresonant systems is a well understood and implemented concept.Obstructions, such as cylindrical rods, can be placed in the near fieldof a rotor to generate an anti-phase secondary sound field that can thenbe tuned to reduce blade tone noise. However, difficulty in tuning theresponse of these interactions often limits their usefulness. Activenoise control approaches have been used for blade tone noise reduction,e.g., introducing active secondary sources into the existing sound fieldof an axial fan. Conventional active approaches have used loudspeakerarrays to reduce levels of fan noise propagating down a duct. Due to theassociated weight and non-compactness of loudspeakers, piezoelectricactuators have been used more recently as acoustic transducers imbeddedinto the stator vanes of axial fans to reduce tonal noise propagations.Air injections, either positioned to generate secondary sources throughinteraction with the rotor blades or used to improve flownon-uniformities generated by a body in a flow field, have been shown toreduce tonal noise. These approaches have proven effective in alaboratory setting, but are generally prohibitively expensive andpotentially unreliable in most actual axial fan applications.

The first known implementation of flow-driven resonator source was togenerate a canceling sound field that reduced fan noise generated by acentrifugal blower. More recently, as disclosed in U.S. Pat. No.7,992,674, a method of using resonators as flow driven secondary sourceshas been developed for axial fans. This method behaves as a quasi-activesource cancellation wherein fluid flow interacts with a resonator as ameans of generating an acoustic source.

The application to propellers, especially open (non-shrouded)propellers, and specifically aircraft propellers has not been addressedfor various reasons. As will be discussed in more detail below, whencancelling propeller tones with an acoustic resonator, the combinationof primary and secondary sound sources tends to create spatial patternsof quiet zones and loud zones. Aircraft applications are somewhat uniquein that the aircraft, in flight, is generally in an acoustic free-field(i.e., no reflective bodies nearby). In fact, the sound that isprojected upward, away from the ground, is usually of no concern.Propellers also have a very directional sound field. As a result, anoise “reduction” solution can be applied that targets the directionalsound field of the propeller but may increase the sound in somedirections.

A key barrier to implementing sound cancellation methods in aircraft isthe added weight. For previous systems, the weight penalty has generallybeen too high for the acoustic performance gains.

It may be desirable to provide a propeller system with a flow-drivenacoustic resonator for modifying or shaping the sound field of apropeller system of an aircraft. It may be desirable to provide a systemwhere the propeller is unaltered and the potential noise reduction in adesired direction is significant. It may also be desirable to provide apropeller sound field modification system that can be integrated intoexisting aircraft structures such that any additional weight or drag dueto the implementation will be relatively nominal. It may be desirable toprovide a secondary sound source that accommodates relative motion ofthe propeller to the aircraft structure with flexible components.

SUMMARY OF THE INVENTION

In one aspect of the disclosure, a propeller system for an aircraftincludes an assembly for modifying a sound field of the propellersystem. The propeller system includes a rotor supported for rotationabout a rotor axis. The rotor has a central hub and a plurality ofblades each extending outwardly from the hub to a tip. The rotor andblades are operable to propel an aircraft to travel in a direction. Therotor blades define a rotor plane perpendicular to the rotor axis. Theblade tips define a circumferential rotational path as the blades arerotated by the rotor. The propeller system includes an acousticresonator having an opening disposed within a distance to the propellerblade tip that is small compared to the wavelength of the propeller'sfundamental blade tone and proximate to the rotor plane. The resonatoris excited by tip flow of the blade as it passes the opening. Theacoustic resonator is configured and positioned so as to direct acousticenergy to modify the sound field of the propeller system at blade passor higher harmonic frequency tones in a desired direction relative tothe aircraft.

According to some aspects, a method for modifying a sound field of apropeller system of an aircraft may include receiving data representingan acoustic profile of a propeller system of an aircraft; receivingreal-time data indicating a propeller speed, orientation of theaircraft, and a weather condition surrounding the aircraft; determininga desired modification to the acoustic profile based on the receivedreal-time data; and tuning an acoustic resonator to achieve the desiredmodification to the acoustic signature of the propeller system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an exemplary propeller system in accordancewith various aspects of the disclosure.

FIG. 2 illustrates how a passing propeller blade tip excites aresonator.

FIG. 3 is a perspective view of an exemplary propeller system inaccordance with various aspects of the disclosure.

FIG. 4 is a top view of an exemplary propeller system in accordance withvarious aspects of the disclosure.

FIG. 5 is a diagrammatic view illustrating a modified sound field of apropeller system in accordance with various aspects of the disclosure.

FIG. 6 is a partially-open side view of an exemplary acoustic resonatorin accordance with various aspects of the disclosure.

FIG. 7 is a graph that shows the directivity of the propeller measuredin the plane of the rotor in the far field showing the variation of thesound pressure level.

FIG. 8 is a graph that shows a fixed point in the far field pressurelevel of the propeller relative to the distance of the resonator openingfrom the blade tip.

FIG. 9 is a side view of an exemplary propeller system in accordancewith various aspects of the disclosure.

FIG. 10 is a perspective view of an exemplary propeller system inaccordance with various aspects of the disclosure.

FIG. 11 is a block diagram of an exemplary system for modifying thesound field of an aircraft in real-time while the vehicle is inoperation.

FIG. 12 is a block diagram of an exemplary system and/or computingdevice for carrying out processes of the apparatus according to variousaspects of the disclosure.

FIG. 13 is a flow chart illustrating an exemplary method for altering asound field of a propeller system of an aircraft according to variousaspects of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to specific embodiments orfeatures, examples of which are illustrated in the accompanyingdrawings. Generally, corresponding or similar reference numbers will beused, when possible, throughout the drawings to refer to the same orcorresponding parts.

The present invention provides an acoustic resonator configuration foruse with or as part of a propeller system so as to provide modificationof propeller fan noise propagation and/or shaping of a propeller soundfield. In some aspects, the acoustic resonator may cancel the entiretyor a substantial portion of the tonal component of noise from thepropeller in one or more directions.

Referring to FIG. 1, a propeller system 110 according to an embodimentof the present disclosure includes a rotor 114 rotatable about a rotoraxis A. As shown, the rotor 114 has a central hub 116 and a plurality ofrotor blades 118 extending outwardly from the hub 116 to tips 120. Therotor blades 118 may be said to define and generally be disposed along arotor plane R. The rotor plane R is generally at the midpoint of therotor blades and perpendicular to the rotor axis A about which the rotorrotates.

The system 110 further includes at least one acoustic resonator 122 as asecondary sound source to modify and/or shape the tonal output of theaxial propeller system 110. For example, in some aspects, the resonator122 may modify or shape the sound field of the propeller system 110 or,in some aspects, attenuate or cancel a component of the tonal output ofthe propeller system 110. The resonator 122 is driven by airflow acrossan opening 124 of the resonator. The airflow is generated by the passingfan blade tips 120 as they are rotatably driven by the rotor 114. Theairflow across the opening 124 causes the resonator 122 to create a toneor sound with a frequency, a phase, and a magnitude. As will be clear tothose of skill in the art, the resonator 122 may be configured to createa tone operable to reduce the blade pass frequency tone of the propellersystem due to noise cancellation between the resonator tone and thepropeller system tone.

As shown in the embodiment of FIG. 1, the resonator 122 may beconfigured as a streamlined, hollow-cavity structure that produces asingle secondary source near the propeller blade tips 120. While theacoustic resonator 122 may take various forms other than shown, theillustrated embodiment uses a closed ended tubular resonator having anopening 124 close enough to the passing rotor blade tips 120 to insurethat the resonator is sufficiently driven by airflow at the appropriatesource strength. Generally, it is counterintuitive to place structuresand/or components close to a propeller, since structures close to thepropeller will tend to make the propeller less efficient and louder.

Referring to FIG. 7, the resultant sound level created by the propellersystem combined with a secondary source located close to the propellerblade tips 120 is illustrated. The sound level is measured in the planeR and in the acoustic far field (a distance of several acousticwavelengths of the fundamental blade tone) of the propeller blades atdifferent angles relative to the secondary source. Note that somedirections yield a decrease in sound pressure, and some yield anincrease in sound pressure. FIG. 8 shows the amplitude of the propellernoise level measured at a location as the amplitude of a secondarysource is changed by moving it farther from the propeller blade tips120. The data point at 0 is the noise level without a secondary source.

FIG. 2 illustrates the mechanism by which such an acoustic resonator isdriven by passing fan blades. The conventional use of resonators isdescribed, for example, in U.S. Pat. No. 6,454,527. According to thepresent disclosure, the passing blade tips 120 generate periodicpressure fluctuations at the mouth or opening 124 of the resonator 122,thereby forcing a resonator response.

Referring now to FIGS. 3 and 4, a pair of resonators 322 are disposedadjacent the circumferential path of the rotor blade tips 120. Theopenings 324 of the resonators 322 may be said to be on opposite sidesof the rotor 114 in the rotor plane R. Alternatively, the resonatoropenings 324 may be positioned differently than shown.

As will be understood by those of skill in the art, the length of theresonators depends on the resonance frequency required to modify and/orshape the resultant propeller sound field. For example, for flow-drivenresonators, preferred resonator lengths are ¼ wavelength and oddmultiples thereof. As known to those of skill in the art, the dominanttone of typical propellers occurs at the blade pass frequency. Theresonators 122, 322 may be tuned so as to provide a sound sourceoperable to cancel at least a portion of the blade pass frequency tonein at least one direction perpendicular and oblique to the flowdirection.

Propeller systems typically have very directional sound fields. As aresult, for aircraft, a noise “reduction” solution can be applied thattargets a directional sound field of the propeller but actuallyincreases the sound in some directions. The spatial distribution of thesound field may be controlled by altering the phase and amplitude of thesecondary sources relative to the unwanted propeller noise. There aredifferent techniques to control the phase and amplitude depending on themethod used to generate the sound. For acoustic resonators, thelocation, damping, shape of, and number of resonant structures willdetermine the secondary sound field that can be generated to modifyand/or shape the spatial noise map of the propeller system.

The acoustic resonators 122, 322 have a resonance frequency which can betuned to be near to the primary blade pass frequency for a maximumresponse. In some embodiments, the resonance frequency is within 10% ofthe blade pass frequency.

FIG. 5 illustrates cancellation of sound waves using a properly tunedsystem. The original sound signal generated by the propeller system 110is shown at 130. The output of the sound source created by the resonator122 is shown at 132. The output of the resonator 122 is nearly inanti-phase with the original sound, thereby cancelling at least aportion of the original signal. The resulting sound wave is shown at134. As will be clear to those of skill in the art, FIG. 5 illustratesthe changes in sound directivity diagrammatically. It can be seen thatthe source reduces the amplitude of the sound in one direction butactually amplifies it in the other.

It should be appreciated by persons of skill in the art that the bladepass frequency of a propeller system depends on the rotational speed ofthe rotor. In many applications the speed is predetermined. That is, thepropeller system is designed such that the rotational speed of the rotoris a constant predetermined speed. For applications such as these, aresonator with a predetermined resonance frequency may be used toprovide resonator configuration in accordance with the presentdisclosure. For example, the resonator configuration may be determinedby a predetermined length of a quarter-wavelength resonator, which ofcourse includes resonance frequencies at ¾ wavelength, 5/4 wavelength,etc., (e.g., odd multiples of ¼). Therefore, for purposes of thisdisclosure, “a quarter-wavelength resonator” is defined as having alength of one-quarter wavelength and any odd multiple thereof.

In other applications, it may be desirable to provide a resonator withadjustable characteristics since the demands on the system will changefor changing flight conditions. Specifically, the system will need to betunable to changing speed of the propeller, orientation of the aircraft,and environmental changes for the greatest practical effectiveness. Thegeometry of the structure of the acoustic resonator must be altered tomatch the changing conditions.

Tuning of acoustic devices in other realms is common—musicalinstruments, mufflers, and cavity resonators used in wall constructionare examples. Some methods for structural tuning include changing theacoustic length by moving the back wall of a tube structure (e.g.,trombone style of tuning); varying hole locations (e.g., flute style oftuning); and varying the number or area of holes. According to variousaspects of this disclosure, two levels of implementation and activationare available: external tuning, where a signal is received by the systemto manually tune the system; or embedded tuning, where the tuning isperformed automatically by embedded electronics and sensors andactuators. An embedded system would entail sensors for propeller speed,air temperature, orientation, and tuning status; a microprocessor tocontrol the system; actuator(s) to adjust the tuning of the structure;an algorithm to translate the sensor readings into an output; and anenergy source.

FIG. 6 illustrates an optional adjusting mechanism 140 at the end of aresonator tube. The adjusting mechanism 140 is operable to adjust theinternal length of the tube. The adjustable mechanism 140 may include anadjustable wall 142 movable by a piston arrangement 144 operable to movethe wall 142 to adjust the impedance of the resonator, as would beunderstood by those of skill in the art. The resonator may include anend wall 150 with a microphone assembly 152 which may be included forfeedback or tuning purposes. The configuration of FIG. 6 may be used forinitially tuning a resonator system or for actively adjusting thecharacteristics of the resonator in operation, such as with a variablespeed propeller system. Other approaches for adjusting the resonancefrequency or other characteristics of the resonators will be understoodby those of skill in the art.

Systems and methods for modifying the acoustic signature of an aircraftwill now be described. U.S. Pat. No. 8,036,821, which is incorporatedherein by reference in its entirety, describes exemplary systems andmethods for modeling the acoustics of a vehicle and the vehicle'ssurroundings.

FIG. 11 is a block diagram of a system 1100 for modifying the acousticsignature of an aircraft in real-time while the vehicle is in operation.System 1100 includes memory 1110, one or more acoustic sensors 1120, oneor more atmospheric conditions sensors 1130, one or more wind sensors1140, and a global positioning system (GPS) 1150, each being coupled toa processor/controller 1160. System 1100 may also include a display 1170coupled to processor 1160.

Memory 1110 may be any hardware, device, logic, and/or firmware capableof storing computer data, computer code, computer-executableinstructions, computer programs, and/or the like for retrieval and/oruse by processor 1160. The acoustic signature of the aircraft'spropeller system, map/geographic data, terrain data, and/or the weatherdata, among other things, may be stored in memory 1110.

Each acoustic sensor 1120 may be any hardware and/or device capable ofdetecting the acoustic profile of the environment surrounding thevehicle while the vehicle is en route to the destination. In oneembodiment, acoustic sensor 1120 comprises a noise detector 2210 and avehicle noise filter 2220. Noise detector 2210 may be any hardwareand/or device capable of detecting sound or noise in the environmentsurrounding the vehicle. Vehicle noise filter 2220 may be any hardwareand/or device capable of filtering out the noise emitted by the vehiclewhile the vehicle is operating in the environment so that the amount ofbackground noise in the environment surrounding the vehicle can bedetermined.

Atmospheric conditions sensor(s) 1130 may be any hardware and/or devicecapable of detecting the actual weather conditions surrounding thevehicle while the vehicle is en route to the destination. For example,atmospheric conditions sensor 1130 may detect air temperature, airdensity, air pressure, turbulence, humidity, precipitation, and/or anyother condition that may amplify, dampen, block, and/or propagate soundemitted from the vehicle.

Each wind sensor 1140 may be any hardware and/or device capable ofdetecting the wind velocity and direction at one or more altitudes whilethe vehicle is en route to the destination. In one embodiment, windsensor 1140 comprises a Light Detection and Ranging (LIDAR) system formeasuring the wind velocity and direction. In another embodiment, windsensor 1140 comprises a Laser Detection and Ranging (LADAR) system formeasuring the wind velocity and direction.

GPS 1150 may be any hardware and/or device capable of detecting thepresent position and tracking the position of the vehicle while thevehicle is en route to the destination. Global positioning systems arewell-known in the art, and therefore, the details of GPS 1150 need notbe described herein.

It should be appreciated that the system 1100 may include additionalconventional sensors for detecting and/or measuring the rotational speedof the propeller, the speed of the aircraft, orientation of theaircraft, and the like. For example, accelerometers, gyroscopes, or thelike may be used to measure the orientation or attitude of the aircraft.In some aspects, the GPS 1150 may be used to measure the speed of theaircraft.

Processor 1160 may be any hardware, device, logic, and/or firmwarecapable of executing computer code, computer instructions, computermodules, and/or computer programs. Processor/controller 1160 is alsoconfigured to receive data from acoustic sensor 1120, atmosphericconditions sensor 1130, wind sensor 1140, and/or GPS 1150 and generate areal-time acoustic profile of the aircraft in relation to thesurrounding area while the vehicle is en route to the destination andtransmit the real-time acoustic profile to display 1170 for presentationto a user. Accordingly, the operator of the vehicle can then optimallytune an acoustic waveguide to modify the acoustic signature of theaircraft so as to maintain the desired directivity of the acoustic fieldgenerated by propeller and resonator in combination.

Referring to FIG. 12, an exemplary system and/or computing device 1200for carrying out processes of the apparatus disclosed herein may includea processing unit (CPU or processor) 1220 and a system bus 1210 thatcouples various system components including the system memory 1230 suchas read only memory (ROM) 1240 and random access memory (RAM) 1250 tothe processor 1220. The system 1200 can include a cache 1222 of highspeed memory connected directly with, in close proximity to, orintegrated as part of the processor 1220. The system 1200 copies datafrom the memory 1230 and/or the storage device 1260 to the cache 1222for quick access by the processor 1220. In this way, the cache providesa performance boost that avoids processor 1220 delays while waiting fordata. These and other modules can control or be configured to controlthe processor 1220 to perform various actions. Other system memory 1230may be available for use as well. The memory 1230 can include multipledifferent types of memory with different performance characteristics. Itcan be appreciated that the disclosure may operate on a computing device100 with more than one processor 1220 or on a group or cluster ofcomputing devices networked together to provide greater processingcapability. The processor 1220 can include any general purpose processorand a hardware module or software module, such as module 1 1262, module2 1264, and module 3 1266 stored in storage device 1260, configured tocontrol the processor 1220 as well as a special-purpose processor wheresoftware instructions are incorporated into the processor. The processor1220 may be a self-contained computing system, containing multiple coresor processors, a bus, memory controller, cache, etc. A multi-coreprocessor may be symmetric or asymmetric.

The system bus 1210 may be any of several types of bus structuresincluding a memory bus or memory controller, a peripheral bus, and alocal bus using any of a variety of bus architectures. A basicinput/output (BIOS) stored in ROM 1240 or the like, may provide thebasic routine that helps to transfer information between elements withinthe computing device 1200, such as during start-up. The computing device1200 further includes storage devices 1260 such as a hard disk drive, amagnetic disk drive, an optical disk drive, tape drive, and othernon-transitory media. The storage device 1260 can include softwaremodules 1262, 1264, 1266 for controlling the processor 1220. The system1200 can include other hardware or software modules. The storage device1260 is connected to the system bus 1210 by a drive interface. Thedrives and the associated computer-readable storage media providenonvolatile storage of computer-readable instructions, data structures,program modules and other data for the computing device 1200. In oneaspect, a hardware module that performs a particular function includesthe software component stored in a tangible computer-readable storagemedium in connection with the necessary hardware components, such as theprocessor 1220, bus 1210, display 1270, and so forth, to carry out aparticular function. In another aspect, the system can use a processorand computer-readable storage medium to store instructions which, whenexecuted by the processor, cause the processor to perform a method orother specific actions. The basic components and appropriate variationscan be modified depending on the type of device, such as whether thedevice 1200 is a small, handheld computing device, a desktop computer,or a computer server.

Although the exemplary embodiment(s) described herein employs the harddisk 1260, other types of computer-readable media which can store datathat are accessible by a computer, such as magnetic cassettes, flashmemory cards, digital versatile disks, cartridges, random accessmemories (RAMs) 1250, read only memory (ROM) 1240, a cable or wirelesssignal containing a bit stream and the like, may also be used in theexemplary operating environment. Tangible computer-readable storagemedia, and a computer-readable storage device, expressly exclude mediasuch as energy, carrier signals, electromagnetic waves, and signals perse.

To enable user interaction with the computing device 1200, an inputdevice 1290 represents any number of input mechanisms, such as amicrophone for speech, a touch-sensitive screen for gesture or graphicalinput, keyboard, mouse, motion input, speech and so forth. An outputdevice 1270 can also be one or more of a number of output mechanismsknown to those of skill in the art. In some instances, multimodalsystems enable a user to provide multiple types of input to communicatewith the computing device 1200. The communications interface 1280generally governs and manages the user input and system output. There isno restriction on operating on any particular hardware arrangement andtherefore the basic hardware depicted may easily be substituted forimproved hardware or firmware arrangements as they are developed.

For clarity of explanation, the illustrative system embodiment ispresented as including individual functional blocks including functionalblocks labeled as a “processor” or processor 1220. The functions theseblocks represent may be provided through the use of either shared ordedicated hardware, including, but not limited to, hardware capable ofexecuting software and hardware, such as a processor 1220, that ispurpose-built to operate as an equivalent to software executing on ageneral purpose processor. For example the functions of one or moreprocessors presented in FIG. 12 may be provided by a single sharedprocessor or multiple processors. (Use of the term “processor” shouldnot be construed to refer exclusively to hardware capable of executingsoftware.) Illustrative embodiments may include microprocessor and/ordigital signal processor (DSP) hardware, read-only memory (ROM) 1240 forstoring software performing the operations described below, and randomaccess memory (RAM) 1250 for storing results. Very large scaleintegration (VLSI) hardware embodiments, as well as custom VLSIcircuitry in combination with a general purpose DSP circuit, may also beprovided.

The logical operations of the various embodiments are implemented as:(1) a sequence of computer implemented steps, operations, or proceduresrunning on a programmable circuit within a general use computer, (2) asequence of computer implemented steps, operations, or proceduresrunning on a specific-use programmable circuit; and/or (3)interconnected machine modules or program engines within theprogrammable circuits. The system 1200 shown in FIG. 12 can practice allor part of the recited methods, can be a part of the recited systems,and/or can operate according to instructions in the recited tangiblecomputer-readable storage media. Such logical operations can beimplemented as modules configured to control the processor 1220 toperform particular functions according to the programming of the module.For example, FIG. 1 illustrates three modules Mod1 1262, Mod2 1264 andMod3 1266 which are modules configured to control the processor 1220.These modules may be stored on the storage device 1260 and loaded intoRAM 1250 or memory 1230 at runtime or may be stored in othercomputer-readable memory locations.

Referring now to FIG. 13, an exemplary method 1300 for altering a soundfield of a propeller system of an aircraft is illustrated. The methodmay employ one of the exemplary apparatuses disclosed herein. The methodcommences at step 1310 and continues to step 1320. In step 1320, thecontroller 1160 determines a desired flight path of the aircraft, andcontrol continues to step 1330.

Then, in step 1330, the controller 1160 determines a real-time acousticprofile of a propeller system of the aircraft, and control continues tostep 1340 where the controller 1160 determines whether the acousticprofile of the propeller system needs to be altered. If, in step 1340,the controller 1160 determines that the acoustic profile needs to bealtered, then control proceeds to step 1350 where the controller 1160operates an acoustic resonator to modify or move the acoustic profile asneeded. Control then returns to step 1330.

If the controller 1160 determines in step 1340 that the acoustic profiledoes not need to be altered, then control returns to step 1330.

As shown in FIG. 1, a resonator consistent with the present disclosuremay be integrated into a fin of an aircraft. FIGS. 3 and 4 illustrateresonators extending from the fuselage of an aircraft. It should beappreciated that resonators may be integrated into struts (FIG. 9) orprovided as a flexible attachment (FIG. 10). It should further beappreciated that the resonators may be coming off the fuselage (theinitial figure in the introduction) integrated into a wing, provided asa separate UAV ‘slice’, or integrated in the tail boom of a helicopter.

As mentioned above, it is counterintuitive to place resonant structuresclose to the propeller, since they will tend to amplify harmonic noise.However, several techniques may be implemented to reduce this effect, sothat a resonant structure can be placed very close to the propeller. Onepossible implementation is the inclusion of variable impedancewalls/sections. For example, a relatively rigid resonator structure mayinclude a rubber section. Another possible implementation is the use ofbranch resonators, which may comprise small tubes extending from theprimary resonator so that the higher harmonics of the quarter wavelengthresonator are no longer integer multiples of the fundamental frequencyof the blade tones. This feature allows the higher harmonic resonatortones to be decoupled from the higher harmonic blades tones and thus notexcited by the passing blade tips. In some aspects, the mouth to theresonator opening may be shaped to alter the harmonic noise; theresonator may include bends; and/or the cross-section or diameter of theresonator may be modified to vary the acoustics of the secondary source.According to some aspects, the height, shape, or material of the backwall of the resonator cavity can be altered. In some aspects, dampingmaterials may be placed in the resonator, and the acoustic damping canbe controlled by the location, type, and amount of damping materials.According to various aspects, the system may include multiple resonatorstuned to different frequencies.

The first advantage of the invention is that a designer can use the mostefficient propeller for the aircraft propulsion system independently ofthe generated noise. The second advantage is a result of the noisecancellation nature of the solution and the collocation of the secondarysources. Noise cancellation techniques can theoretically eliminate tonalnoise sources completely when the secondary source is collocated withthe noise source.

From the foregoing, it will be appreciated that, although specificembodiments have been described herein for purposes of illustration,various modifications or variations may be made without deviating fromthe spirit or scope of inventive features claimed herein. Otherembodiments will be apparent to those skilled in the art fromconsideration of the specification and figures and practice of thearrangements disclosed herein. It is intended that the specification anddisclosed examples be considered as exemplary only, with a trueinventive scope and spirit being indicated by the following claims andtheir equivalents.

What is claimed is:
 1. A method for modifying a sound field of apropeller system of an aircraft, the propeller system including a rotorhaving a central hub and a plurality of blades extending from the hub toa tip, the method comprising: positioning a tunable acoustic resonatoron the aircraft such that the acoustic resonator has an opening disposedproximate the blade tips and proximate a rotor plane; receiving datarepresenting an acoustic signature of the propeller system of theaircraft; receiving real-time data relative to the aircraft in flightindicating a propeller speed, orientation of the aircraft, and anenvironmental condition surrounding the aircraft; generating a real-timeacoustic profile of the aircraft in flight based on the acousticsignature and the received real-time data; and tuning the acousticresonator based on the real-time acoustic profile to achieve an acousticfield generated by the resonator and the propeller system of thein-flight aircraft in combination having a desired directivity based onthe real-time acoustic profile.
 2. The method of claim 1, furthercomprising displaying the acoustic field of the aircraft.
 3. The methodof claim 1, wherein the step of tuning includes directing acousticenergy, via the acoustic resonator having an opening disposed within adistance to the propeller blade tip that is smaller than the wavelengthof the propeller's fundamental blade tone and proximate a rotor plane,so as to modify the sound field of the propeller system at blade pass orhigher harmonic frequency tones.
 4. The method of claim 3, wherein theacoustic resonator comprises a one-quarter wave resonator, and theopening is disposed within one-eighth acoustic wavelength of thecircumferential rotational path of the blade tips.
 5. The method ofclaim 3, further comprising: directing acoustic energy, via at least oneadditional acoustic resonator spaced apart from the acoustic resonatorabout the circumferential rotational path of the blade tips in a planetransverse to the rotor axis and having an opening disposed within adistance to the propeller blade tip that is smaller than the wavelengthof the propeller's fundamental blade tone and proximate the rotor plane,so as to modify the sound field of the aircraft propeller system atblade pass or higher harmonic frequency tones.
 6. The method of claim 3,wherein the directing step comprises directing acoustic energy to modifythe sound field of the propeller system at blade pass or higher harmonicfrequency tones in a direction between the aircraft and a targetlocation.
 7. A method for altering a sound field of an aircraft whilemaintaining a desired flight path of the aircraft, the methodcomprising: positioning a tunable acoustic resonator on the aircraftsuch that the acoustic resonator has an opening disposed proximate theblade tips and proximate a rotor plane; receiving real-time datarelative to the aircraft in flight along the desired flight pathindicating a propeller speed, orientation of the aircraft, and anenvironmental condition relative to the desired flight path; generatinga real-time acoustic profile of a propeller system of the aircraft basedon the real-time data; and tuning the acoustic resonator based on thereal-time acoustic profile to achieve an acoustic field generated by theresonator and the propeller of the in-flight aircraft in combinationhaving a desired directivity based on the desired flight path.
 8. Themethod of claim 7, wherein the environmental condition includes groundfeatures along the desired flight path of the aircraft.
 9. The method ofclaim 8, further comprising a step of modifying or moving the acousticfield such that sound from the propeller system is directed away fromobserved ground features.
 10. The method of claim 7, wherein the step ofgenerating a real-time acoustic profile includes: receiving datarepresenting an acoustic profile of a propeller system of an aircraft;and receiving real-time data indicating a propeller speed, orientationof the aircraft, and a weather condition surrounding the aircraft. 11.The method of claim 7, wherein the step of tuning an acoustic resonatorincludes directing acoustic energy, via the acoustic resonator having anopening disposed within a distance to the propeller blade tip that issmaller than the wavelength of the propeller's fundamental blade toneand proximate a rotor plane, so as to achieve a determined movement ormodification of the sound field of the propeller system.
 12. A systemfor altering a sound field of an aircraft while maintaining a desiredflight path of the aircraft, the system comprising: a tunable acousticresonator disposed on the aircraft such that the acoustic resonator hasan opening disposed proximate the blade tips and proximate a rotorplane; and a controller configured to receive real-time data relative toan environmental condition relative to the desired flight path; generatea real-time acoustic profile of a propeller system of the aircraft basedon the real-time data; and tune the acoustic resonator based on thereal-time acoustic profile to achieve an acoustic field generated by theresonator and the propeller of the in-flight aircraft in combinationhaving a desired directivity based on the desired flight path.
 13. Thesystem of claim 12, wherein the environmental condition includes groundfeatures along the desired flight path of the aircraft.
 14. The systemof claim 13, wherein the controller is further configured to modify ormove the acoustic field such that sound from the propeller system isdirected away from observed ground features.
 15. The system of claim 12,wherein the controller being configured to generate a real-time acousticprofile includes: receiving data representing an acoustic profile of thepropeller system of the aircraft; and receiving real-time dataindicating a propeller speed, orientation of the aircraft, and a weathercondition surrounding the aircraft.
 16. The system of claim 12, whereinthe controller being configured to tune the acoustic resonator includesdirecting acoustic energy, via the acoustic resonator having an openingdisposed within a distance to the propeller blade tip that is smallerthan the wavelength of the propeller's fundamental blade tone andproximate a rotor plane, so as to achieve the determined movement ormodification of the sound field of the propeller system.